The mitral valve is a functional unit composed of multiple dynamically interrelated units. During cardiac cycle, the fibrous skeleton, the anterior and posterior leaflets, the papillary muscles, the chordae tendinea, and the ventricular and atrial walls all interplay symphonically to render a competent valve. The complex interaction between the mitral valve and the ventricle by the subvalvular apparatus (the papillary muscles and the chordae tendinea) is essential in that it maintains the continuity between the atrio-ventricular ring (which is part of the fibrous skeleton of the heart) and the ventricular muscle mass, which is essential for the normal function of the mitral valve.
The chordae tendinea, which connect the valve leaflets to the papillary muscles (PM) act like “tie rods” in an engineering sense. Not only do the chordae tendinea prevent prolapse of the mitral valve leaflets during systole, but they also support the left ventricular muscle mass throughout the cardiac cycle.
To function adequately, the mitral valve needs to open to a large orifice area and, for closure, the mitral leaflets need to have an excess of surface area (i.e. more than needed to effectively close the mitral orifice). On the other hand, systolic contraction of the posterior ventricular wall around the mitral annulus (MA) creates a mobil D-shaped structure with sphincter-like function which reduces its area by approximately 25% during systole, thus exposing less of the mitral leaflets to the stress of the left ventricular pressure and flow.
Although the primary function of the mitral valve is to act as a one-way no return valve, it has been postulated that the structural integrity of the MA-PM continuity is essential for normal left ventricular function.
Since it was first suggested in the mid-1960's that preservation of the subvalvular apparatus during mitral valve replacement might prevent low cardiac output in the early postoperative period, this important observation was largely overlooked by most surgeons for many years.
There is now considerable laboratory and clinical evidence to corroborate this position, as evidence has demonstrated that chordal excision is associated with a change in left ventricular shape from oval to spherical, which can lead to a significant increase in postoperative left ventricular end systolic volume and wall stress, along with a decline in ejection fraction.
The majority of evidence appears to support the concept that preservation of the subvalvular apparatus with the MA-PM continuity in any procedure on the mitral valve is important for the improved long-term quality and quantity of life after mitral valve surgery. Reparative techniques to correct mitral valve disease are often the best surgical approach for dealing with mitral valve abnormalities, however mitral valvuloplasty is not always feasible because of extensive fibrosis, leaflets calcification, or massive chordal rupture. Mitral valve replacement using either a mechanical valve or a bioprosthetic valve thus remains the best surgical solution for severe mitral valve disease.
However, there are many additional problems that face patients after valve replacement with a prosthetic valve. Valve-related problems include limitation of the mitral flow (due to a small effective orifice area) during exercise and high cardiac output imposed by a smaller size artificial valve as compared with the natural valve orifice area.
Further, the rigid structure of an artificial valve prevents the physiologic contraction of the posterior wall of the left ventricle surrounding the MA during systole. Surgical interruption of the MA-PM continuity accounts for changes in geometry mechanics and performance of the left ventricle. Myocardial rupture, a lethal complication of mitral valve replacement, results from excision or stretching of the papillary muscle in a thin and fragile left ventricle. Myocardial rupture can also be caused by a strut of a stented bioprosthetic valve eroding into or protruding through the posterior left ventricle wall. Maintaining the MA-PM continuity appears to provide a substantial degree of protection from this devastating complication. Also, the difficulties in controlling adequate anticoagulation for a mechanical valve bring a high morbidity risk factor of thromboembolic and hemorragic complication and endocarditis.
Stented tissue valves, although less thrombogenic, are not reliably durable and, because of the rigid stent, they are less hemodynamically efficient. Stentless valves are considered to have the potential advantages of superior hemodynamic performance and enhanced durability and have already showed satisfactory mid-term results in the aortic position. From these points of view, it is expected that new stentless valves in the mitral position will be developed. However, stentless mitral valves are not yet commonly available for clinical use because of the anatomical and functional complexity of the mitral valve and the subvalvular apparatus, resulting in the difficulties of the design and implantation procedures of the stentless mitral valves. The present invention provides and apparatus and method for replacing a native mitral valve with a stentless, bioprosthetic valve that maintains the anatomical and functional complexity of the mitral valve and the subvalvular apparatus.